Plateau of practice effects and noise with repeat SDMT testing in multiple sclerosis

The Symbol Digit Modalities Test (SDMT) is the most widely used test of cognition in people with multiple sclerosis (PwMS), and repeated testing is confounded by test-retest noise and practice effects.

To investigate the extent to which SDMT practice effects build and plateau with high-frequency testing, reliable cutoffs for longitudinal change, and whether short-interval testing improves detection of cognitive decline.

PwMS were tested with the SDMT monthly across 3 years. Plateau regression analyses were used to determine inflection points of practice effects, and the reliable-change was assessed. To evaluate effects of testing density on cognitive decline detection, this sample was compared with a separate low-density testing group.

The study included 71 people with relapsing-remitting MS (77.5% female), mean (standard deviation (SD)) age 37.3 (9.3), with 27.8 (21.3) SDMT assessments over 3.2 (2.4) years. The plateau of practice effects was reached after 18 repetitions (p < 0.001). Within this sample, ⩾7-point SDMT change was needed to detect cognitive decline with 90% confidence. Higher testing density did not improve detection of cognitive decline (p = 0.256).

We observed practice effects building for eighteen SDMT assessments and test-retest variability consistent with literature. These results provide guidance on SDMT which should be accounted for with alternate versions and reliable-change methodologies.